Abstract
The New Madrid seismic zone of the central Mississippi River valley has been interpreted to be a right-lateral strike-slip fault zone with a left stepover restraining bend (Reelfoot reverse fault). This model is overly simplistic because New Madrid seismicity continues 30 km southeast of the stepover. In this study we have analyzed 1704 earthquake hypocenters obtained between 1995 and 2006 in three-dimensional (3-D) space to more accurately map fault geometry in the New Madrid seismic zone. Most of the earthquakes appear to align along fault planes. The faults identifi ed include the New Madrid North (29°, 72° SE), Risco (92°, 82° N), Axial (46°, 90°), Reelfoot North (167°, 30° SW), and Reelfoot South (150°, 44° SW) faults. A diffuse zone of earthquakes exists where the Axial fault divides the Reelfoot fault into the Reelfoot North and Reelfoot South faults. Regional mapping of the top of the Precambrian crystalline basement indicates that the Reelfoot North fault has an average of 500 m and the Reelfoot South fault 1200 m of down-to-the-southwest normal displacement. Since previously published seismic refl ection profi les reveal reverse displacement on top of the Paleozoic and younger strata, the Reelfoot North and South faults are herein interpreted to be inverted basement normal faults. The Reelfoot North and Reelfoot South faults differ in strike, dip, depth, and displacement, and only the Reelfoot North fault has a surface scarp (monocline). Thus, the Reelfoot fault is actually composed of two left-stepping restraining bends and two faults that together extend across the entire width of the Reelfoot rift.
Highlights
The New Madrid seismic zone, beneath the central Mississippi River valley, is undergoing considerable study as it is the most seismically active area in the eastern United States and because it experienced at least three devastating earthquakes in 1811–1812 (Nuttli, 1982; Penick, 1981; Johnston, 1996; Johnston and Schweig, 1996; Tuttle et al, 2002) (Fig. 1)
A surface scarp due to Reelfoot fault propagation folding (Champion et al, 2001) can be traced from the southwestern margin of Reelfoot Lake, Tennessee, where it is ~3 m high north to a maximum of 10 m where the scarp is truncated by the Mississippi River, to less than 3 m high where the scarp is again truncated by the Mississippi River near New Madrid, Missouri (Van Arsdale et al, 1999) (Fig. 3)
Seismic reflection profiles reveal that the Reelfoot fault displaces the post-Paleozoic section ~70 m in a reverse sense (Sexton, 1988; Van Arsdale et al, 1998; Champion et al, 2001)
Summary
The New Madrid seismic zone, beneath the central Mississippi River valley, is undergoing considerable study as it is the most seismically active area in the eastern United States and because it experienced at least three devastating earthquakes in 1811–1812 (Nuttli, 1982; Penick, 1981; Johnston, 1996; Johnston and Schweig, 1996; Tuttle et al, 2002) (Fig. 1) This seismicity has been attributed to reactivation of basement faults within the subsurface Cambrian Reelfoot rift (Zoback, 1979; Kane et al, 1981; Braile et al, 1986; Thomas, 1989; Dart and Swolfs, 1998). These high rates have been explained as Holocene initiation of the New Madrid seismic zone or as a burst of Holocene seismic activity on an old fault (Pratt, 1994; Schweig and Ellis, 1994; Van Arsdale, 2000)
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